Legacy mercury and stoichiometry with C, N, and S in soil, pore water, and stream water across the upland-wetland interface: The influence of hydrogeologic setting

Mechanisms influencing retention, biogeochemical cycling, and release of legacy mercury within soils of forests and wetlands remain poorly understood. We quantified mercury pool size and stoichiometry with carbon, nitrogen, and sulfur across forest-wetland transects and among wetlands of different hydrogeologic settings in the Adirondack region of New York State. Average total mercury pool size in soils (to 50 cm depth) was greater in forests (17.5 mg/m²) than in wetlands (6.1 mg/m²; p < 0.010). The average mercury pool size (to 50 cm depth) in shallow-peat riparian wetlands (9.3 mg/m²) was greater than in deep-peat riparian (5.4 mg/m ²; p = 0.099) or headwater wetlands (3.6 mg/m²; p = 0.046). Accumulation of mercury was enhanced at the forest-wetland interface. In mineral horizons of the forest soil and in shallow-peat riparian wetlands, mercury was positively correlated with carbon (r² = 0.73-0.96) and nitrogen (r² = 0.82-0.93), but not sulfur. In contrast, mercury and sulfur were strongly correlated in headwater wetland peat (r² = 0.73). Dissolved mercury was correlated with dissolved organic carbon (DOC) in pore water and stream water of deep-peat and shallow-peat riparian wetlands (r² = 0.46-0.73), but not in headwater wetland pore water. In headwater outlet streams, dissolved mercury was correlated with DOC (r ² = 0.62), but the slope was only one third that in riparian streams. Hydrogeologic setting influences decomposition processes, biogeochemical cycling of mercury, and hydrologic transport that in turn, govern the size and stoichiometry of mercury pools across the upland-wetland interface and among different wetland types. Ultimately, mobilization of legacy mercury into aquatic ecosystems from forest soils and wetlands likely depends upon decomposition dynamics and hydrologic flow paths. Key Points Accumulation of Hg in wetlands was enhanced at the forest-wetland interfaceWetland hydrogeologic setting influences Hg stoichiometry of soils & streamsHg:C ratio of soil & pore water sources may influence Hg:C ratio of stream water